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Description: This interrupt is normally used by the first serial port, COM1. On PCs
that do not use a PS/2-style mouse, this port (and thus this interrupt) are almost always
used by the serial mouse. IRQ4 is also the default interrupt for the third serial port,
COM3, and a popular option for modems, sound cards and other devices. Modems sometimes
come pre-configured to use COM3 on IRQ4.

Conflicts: Conflicts on IRQ4 are relatively common, although not as common as on
IRQ3. On systems that do not use a serial mouse, problems are less common, because COM1
isn't automatically busy whenever the mouse is in use. The two biggest problem areas are
modems that attempt to use COM3/IRQ4 and clash with COM1, and systems that attempt to use
both COM1 and COM3 simultaneously on this same interrupt line. In most cases the problem
can be avoided by changing the conflicting device to a different interrupt (IRQ2 and IRQ5
usually being the best choices). If a PS/2 mouse is being used, you can disable the
built-in COM1 port in the BIOS setup, which will allow a modem to stay at COM3/IRQ4
without causing any problems. However, this is not really recommended. More general
solutions to these issues can be found in the conflict resolution area of
the Troubleshooting Expert.